We have recently shown that no significant differences exist in basal metabolic rate (BMR) or total daily energy expenditure (TDEE) expressed per kg of fat-free mass (FFM) between ambulatory obese and non-obese adolescents. We have furthermore shown no significant differences in BMR, the thermic effects have furthermore shown no significant differences in BMR, the thermic effects of food (TEF), TDEE, or the energy cost of activity (TDEE/BMR), compared to pre-adolescent girls who do not become obese over a five-year period. The girls will be recruited from 9 and 10 year old girls in the Cambridge Public Schools. A cohort of 100 girls at high risk to develop obesity will be established from non-obese pre-adolescent girls with 1 or 2 obese parents. The low risk cohort will consist of 100 non-obese pre-asdolescent girls with neither parent obese. BMR, TDEE and the energy cost of activity (TDEE/BMR) will be measured during the first two years of the study suing indirect calorimetry and the doubly labeled water method (2h2018). The interrelationships of reported activity and V02 max with TDEE/BMR will be examined. The effect of television on BMR will be measured to determined if television viewing reduces resting energy expenditure. Dietary intake will be measured by a 14-day diary to calculate TDEE. A food frequency questionnaire will be used to assess patterns of food intake related to season, TV, and obesity, and will be validated by the dietary diaries. The influence of these variables on the incidence of obesity and fatness will be assessee prospectively. Lipoprotein levels will be measured to determine whether alternations in HDL and LDL are consequences of fitness, activity, or fatness. These data will be allow us to determine whether cohorts at higher risk for the development of obesity have a reduced energy expenditure, and whether a higher incidence of obesity occurs among pre-obese adolescent girls with reduced energy expenditure.